1. Field of the Invention
This invention relates to a method and apparatus for producing a corrugated tube made of synthetic resin, and more particularly to a method and apparatus for manufacturing a corrugated tube the wall of which is corrugated in longitudinal cross-section to provide enhanced flexibility and a high pressure resistance.
2. Prior Art
Various types of flexible hose or tubes made of synthetic resin and method and apparatus for manufacturing the same have been heretofore proposed and some of them are currently in use.
One of the commonly practised prior art methods is to pass a softened synthetic resin tube into mold means having inner corrugated surfaces and then apply pressure to the interior of said tube so as to expand the tube wall against the inner corrugated surfaces of the mold means into the corresponding corrugated configuration to thus produce a flexible tube.
To carry out the foregoing operation on a continuous basis it has been proposed to provide said mold means in the form of a pair of two mold halves divided along a central longitudinal plane passing through the mold means, each mold half comprising a string of interconnected mold half sections in an endless loop. The two sets of mold half sections are adapted to run in their respective endless paths and are arranged such that at least one of the mold half sections of one set is mated with at least one of the mold half sections of the other set. Pressure is applied to the interior of a softened synthetic resin tube as the latter is continuously fed into the mold defined by said mated mold half sections to press the tube wall against the inner corrugated wall of the mold half sections to thus produce a corrugated tube in a continuous manner.
However, this prior art method requires extremely large and complicated equipment, resulting in an increase in the manufacturing cost of the product.
Another form of the prior art process of producing a flexible tube of synthetic resin is to employ a plurality of smooth surfaced roller mandrels rotatable about their own axes and arranged parallel to each other with the axes of the roller mandrels arrayed in a circle so that the mandrels cooperate to define an imaginary cylinder. As a strip of synthetic resin material is continuously fed in a softened state onto said rolled mandrels rotating at an equal speed, it is helically wrapped around the imaginary cylinder defined by said mandrels with portions of adjacent turns of the strip overlapping one on another and joined together to produce a continuous tube. This method involves a reasonable manufacturing cost as compared with the first mentioned method. However, the flexible tube produced according to this type of prior art method required a reinforcing core wire such as metal wire which may be embedded in the overlapping portions of the strip as it is spirally wrapped around, in order to impart required flexibility and resistance to exterior pressure. Alternatively, it was necessary to have a portion of the strip thickened with relatively thinned portions joined together in overlapping relation whereby said thickened portion provides adequate strength to withstand exterior pressure.
However, the tube having a core wire embedded in its wall not only required complicated manufacturing procedures but also had the disadvantage that a uniform resistance to exterior pressure was not necessarily ensured throughout the tube wall due to differentials in strength between the core wire-embedded portions and the remaining portions. The tube reinforced with preformed thickened portions also required an additional step of providing such preformed strip of synthetic resin, and it again had the disadvantage that the alternating thickened and thinned portions caused uneveness in thickness resulting in nonuniform strength.